diff --git a/cores/arduino/HardwareSerial.cpp b/cores/arduino/HardwareSerial.cpp
index 9f5ed50..d6d7b60 100644
--- a/cores/arduino/HardwareSerial.cpp
+++ b/cores/arduino/HardwareSerial.cpp
@@ -265,7 +265,7 @@ HardwareSerial::HardwareSerial(ring_buffer *rx_buffer, ring_buffer *tx_buffer,
 
 // Public Methods //////////////////////////////////////////////////////////////
 
-void HardwareSerial::begin(long baud)
+void HardwareSerial::begin(unsigned long baud)
 {
   uint16_t baud_setting;
   bool use_u2x = true;
diff --git a/cores/arduino/HardwareSerial.h b/cores/arduino/HardwareSerial.h
index d229373..eefdcbe 100644
--- a/cores/arduino/HardwareSerial.h
+++ b/cores/arduino/HardwareSerial.h
@@ -49,7 +49,7 @@ class HardwareSerial : public Stream
       volatile uint8_t *ucsra, volatile uint8_t *ucsrb,
       volatile uint8_t *udr,
       uint8_t rxen, uint8_t txen, uint8_t rxcie, uint8_t udrie, uint8_t u2x);
-    void begin(long);
+    void begin(unsigned long);
     void end();
     virtual int available(void);
     virtual int peek(void);